Using the free-living soil nematode C. elegans as a model, we plan to investigate mechanisms by which cell fates are determined in development, both early in embryogenesis when determination appears to be dictated by internally segrating factors, and later when it appears dictated by positional cues. Our specific aims include the following: 1) We will isolate and characterize by microscopy both mutagen-induced suppressible (amber) and Gamma-ray-induced unconditional embryonic lethal mutants, in order to identify classes of pattern defects and thereby better understand the genetic control of pattern formation in early embryos. 2) We will test the possibility of using sensitivity to psoralen-UV treatment as an assay for chromatin structure changes in early cell determination. 3) Using cloned retroviral oncogene probes, we will identify homologous sequences in a C. elegans genomic clone library, assign them to linkage groups, and determine their patterns of expression during embryogenesis to explore the possibility that these genes are developmentally important components of intercellular signalling systems. 4) Using immunofluorescence we will assay for known peptide-hormone and growth-factor analogues in C. elegans and identify their times of appearance during development, spatial distribution, and cells of origin. We will attempt similar analyses of the cognate receptors, to establish patterns of inducing cells and target cells during development. 5) We will attempt to achieve genotypic transformation of C. elegans by microinjection, into maternal ovaries or early zygotes, of various appropriate DNA vectors carrying suppressor-tRNA or drug-resistance genes for which we can select strongly.